PSCA is a cell surface glycoprotein over-expressed in most localized prostate cancer and highly over- expressed in the great majority of prostate cancer bone metastases. Minibodies are antibody fragments with rapid tumor penetration and faster blood clearance than intact antibodies which allows I-124 labeled minibodies to be used as high contrast PET imaging agents. These studies seek to evaluate the ability of the anti-PSCA A11 minibody to address major unmet needs in the field of prostate cancer: non-invasive staging of prostate cancer and the evaluation of treatment response. Aim 1: Evaluate the sensitivity of I-124 labeled A11 minibody immunoPET for detecting models of prostate cancer metastasis and in vivo quantitation of PSCA expression. We will evaluate the sensitivity of the minibody to image small tumors and metastases in lymph node lung and bone and compare this sensitivity with conventional imaging techniques. We will also investigate both the minimum PSCA antigen density needed for A11 minibody immunoPET imaging and the quantitative relationship between A11 uptake and PSCA expression in a tumor. Aim 2: Evaluate the ability of A11 minibody immunoPET to track progression from HGPIN, to invasive adenocarcinoma, to metastases in transgenic mouse models. In this aim, we will use a transgenic prostate cancer mouse model to evaluate the ability of the A11 anti-PSCA minibody to image local prostate cancer, tumor invasion, and tumor metastasis in vivo. Aim 3: Evaluate the ability of A11 minibody immunoPET to non-invasively image tumor response to therapy. In this aim, we will measure changes in tumor cell surface expression of PSCA in response to therapy and evaluate the ability of A11 minibody immunoPET to measure these changes non-invasively as a means of monitoring therapeutic efficacy.